Logs: freenode/#haskell

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2020-11-20 08:46:48	<guest1120>	lex, happy, parsec, they're same?
2020-11-20 08:47:09	<tdammers>	mif suggests Monoid m => Bool -> m -> m -> m, or maybe even Monoid m => Bool -> m -> m -> m -> m
2020-11-20 08:47:16	<merijn>	tdammers: Yeah, but that leads to confusion with monad's and whenM, etc.
2020-11-20 08:47:37	<tdammers>	it's very similar to when from Control.Monad though, isn't it
2020-11-20 08:48:04	<merijn>	yeah
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2020-11-20 08:48:15	<tdammers>	in fact, if Monad could have the Monoid instance it deserves, it would be the exact same thing
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2020-11-20 08:49:32	<dminuoso>	guest1120: Presumably by lex you meant alex?
2020-11-20 08:49:46	<dminuoso>	guest1120: At any rate, they are all parsing/lexing libraries but with different ergonomics and properties.
2020-11-20 08:50:03	<guest1120>	dminuoso: which one is easier?
2020-11-20 08:50:22	<dminuoso>	"easier" is hard to answer in general
2020-11-20 08:50:23	<merijn>	kritzefitz: Some example uses: https://github.com/haskell/cabal/blob/master/Cabal/src/Distribution/Simple/Program/GHC.hs#L203-L209 also note that from/to in the surrounding code https://github.com/haskell/cabal/blob/master/Cabal/src/Distribution/Simple/Program/GHC.hs#L174-L199 are just slightly specialised versions of that function
2020-11-20 08:50:48	<guest1120>	I'm learing parsec by write yourself a scheme with 48 hours
2020-11-20 08:50:52	<merijn>	kritzefitz: It's great for selectively including bits and pieces in complex compound structures
2020-11-20 08:50:58	<guest1120>	but some codes are old
2020-11-20 08:50:59	<dminuoso>	But `megaparsec` is a safe default for a parsing library
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2020-11-20 08:51:26	<dminuoso>	That is, if you want a tl;dr for "which parsing library should I use", you generally cant go wrong with megaparsec.
2020-11-20 08:51:44	<tdammers>	also megaparsec > parsec in 99.9% of cases these days
2020-11-20 08:52:08	<guest1120>	but parsec comes with ghc?
2020-11-20 08:52:20	<tdammers>	does it? I don't think it does
2020-11-20 08:52:28	<koz_>	tdammers: Isn't parsec a boot library?
2020-11-20 08:52:41	<tdammers>	I thought it wasn't, but maybe it is by now
2020-11-20 08:52:53	<guest1120>	Text.Parsec?
2020-11-20 08:53:03	<kritzefitz>	merijn, It does seem useful. But I just realised thta I rarely use a lot of Monoids and now I'm wondering if I'm missing out.
2020-11-20 08:53:18	<koz_>	kritzefitz: You definitely are.
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2020-11-20 08:53:35	<koz_>	You can get a remarkable range of behaviour out of foldMap by varying up your Monoids.
2020-11-20 08:54:08	<dminuoso>	guest1120: The boot libraries should not be a relevant factor for what you should use.
2020-11-20 08:54:24	<merijn>	kritzefitz: You are :p
2020-11-20 08:54:29	<dminuoso>	The fact that GHC comes with them is not for your convenience.
2020-11-20 08:54:45	<merijn>	kritzefitz: Note that the entire surrounding 150-200 lines of code I just linked is basically all composing a bunch of monoids together :p
2020-11-20 08:55:07	<mikevdg>	Haskell: A new way to break your brain every day. `mempty :: Sum Int` --> Sum {getSum = 0}
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2020-11-20 08:56:15	<merijn>	mikevdg: Don't forget Dual
2020-11-20 08:56:18	<guest1120>	ok
2020-11-20 08:56:28	<mikevdg>	merijn: Dual?
2020-11-20 08:56:28	<dminuoso>	Mmm, why exactly does GHC ship with these boot packages at all? Is ghc linked dynamically?
2020-11-20 08:56:40	<mikevdg>	:t Dual
2020-11-20 08:56:41	<lambdabot>	a -> Dual a
2020-11-20 08:56:47	<koz_>	dminuoso: I think they're needed to build GHC itself?
2020-11-20 08:57:02	<mikevdg>	Is there a thing in, e.g. ghci, to also get a bit of documentation on a type?
2020-11-20 08:57:04	<koz_>	There's also Down.
2020-11-20 08:57:09	<merijn>	mikevdg: "newtype Dual a = Dual a" where "mempty = Dual mempty" and "mappend (Dual x) (Dual y) = Dual (mappend y x)"
2020-11-20 08:57:09	<koz_>	@hoogle Dual
2020-11-20 08:57:10	<lambdabot>	Data.Monoid newtype Dual a
2020-11-20 08:57:10	<lambdabot>	Data.Monoid Dual :: a -> Dual a
2020-11-20 08:57:10	<lambdabot>	Data.Semigroup newtype Dual a
2020-11-20 08:57:10	<kritzefitz>	The first thing that comes to minds, when I think about monoids, is that WriterT uses Monoids for the written things. So if I want to collect elements into a list it degrades (:) to (++) and that bother me. But I guess I'm just thinking about this the wrong way.
2020-11-20 08:57:13	<mikevdg>	Or do I have to go to Hoogle?
2020-11-20 08:57:28	<merijn>	> [1,2,3] <> [4,5,6]
2020-11-20 08:57:30	<lambdabot>	[1,2,3,4,5,6]
2020-11-20 08:57:34	<merijn>	> Dual [1,2,3] <> Dual [4,5,6]
2020-11-20 08:57:37	<lambdabot>	Dual {getDual = [4,5,6,1,2,3]}
2020-11-20 08:57:41	<koz_>	:D
2020-11-20 08:57:56	<koz_>	Another cool Monoid: Ordering.
2020-11-20 08:58:00	<merijn>	kritzefitz: There's more monoids than lists, though
2020-11-20 08:58:11	<merijn>	koz_: You're forgetting the best Monoid
2020-11-20 08:58:15	<koz_>	Lists are actually a very uninteresting monoid.
2020-11-20 08:58:18	<dminuoso>	koz_: Oh mmm. I was somehow under the impression that a GHC installation somehow came with these packages
2020-11-20 08:58:22	<koz_>	merijn: (Monoid b) => a -> b?
2020-11-20 08:58:29	<merijn>	koz_: Of course
2020-11-20 08:58:56	<koz_>	newtype MonoidInjection a b :P
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2020-11-20 08:59:12	<guest1120>	why there're <> and ++ two ways to do the same thing?
2020-11-20 08:59:24	<merijn>	:t (++)
2020-11-20 08:59:25	<dminuoso>	guest1120: (<>) is more general than ++
2020-11-20 08:59:25	<lambdabot>	[a] -> [a] -> [a]
2020-11-20 08:59:27	<kritzefitz>	merijn, sure there are. But I guess I'm missing the familiarity with Monoids to recognize when their properties would actually be useful to me.
2020-11-20 08:59:29	<merijn>	:t (<>)
2020-11-20 08:59:29	<koz_>	guest1120: (++) is list-only, while <> will work for any Semigroup.
2020-11-20 08:59:30	<lambdabot>	Semigroup a => a -> a -> a
2020-11-20 08:59:40	<Martinsos>	dminuoso: So specifically for that example with shapes, which one do you think is a better approach? Data types or type classes? And why?
2020-11-20 09:00:00	<merijn>	kritzefitz: Note that the monoid that koz_ mentioned "instance Monoid m => Monoid (r -> m)" applies recursively :)
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2020-11-20 09:00:09	<merijn>	:t comparing fst
2020-11-20 09:00:10	<lambdabot>	Ord a => (a, b) -> (a, b) -> Ordering
2020-11-20 09:00:20	<merijn>	:t comparing snd <> comparing fst
2020-11-20 09:00:22	<lambdabot>	(Ord a1, Ord a2) => (a2, a1) -> (a2, a1) -> Ordering
2020-11-20 09:00:27	<dminuoso>	Martinsos: It's hard to say with that little information.
2020-11-20 09:00:34	<koz_>	It's quite context-specific.
2020-11-20 09:01:06	<merijn>	> sortBy (comparing fst) [(1,2), (2,2), (1,1), (2,1)]
2020-11-20 09:01:08	<lambdabot>	[(1,2),(1,1),(2,2),(2,1)]
2020-11-20 09:01:19	<Martinsos>	dminuoso: you mean to say it depends on which extension is important for us, what we might need in the future?
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2020-11-20 09:01:47	<merijn>	> sortBy (comparing snd) [(1,2), (2,2), (1,1), (2,1)]
2020-11-20 09:01:47	<merijn>	> sortBy (comparing <> comparing snd) [(1,2), (2,2), (1,1), (2,1)]
2020-11-20 09:01:47	<merijn>	eh, whoops
2020-11-20 09:01:47	<merijn>	> sortBy (comparing fst <> comparing snd) [(1,2), (2,2), (1,1), (2,1)]
2020-11-20 09:01:50	<lambdabot>	[(1,1),(1,2),(2,1),(2,2)]
2020-11-20 09:01:50	<lambdabot>	[(1,1),(2,1),(1,2),(2,2)]
2020-11-20 09:01:50	<lambdabot>	error:
2020-11-20 09:01:50	<lambdabot>	• Couldn't match type ‘(a, a1) -> Ordering’ with ‘Ordering’

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